1. Field of the Invention
This invention relates to an actuator having two pistons located in one cylinder and driven by separate drive sources.
More particularly, the invention relates to an actuator having a first piston and a second piston inserted in one cylinder, the second piston including a piston rod located for relative sliding movement along a piston rod of the first piston. The cylinder having a first cylinder chamber for the first piston and a second cylinder chamber for the second piston formed separately and divided by a partition wall, one of the first and second cylinder chambers being connected to an oil pressure source and the other cylinder chamber being connected to an air pressure source while the first and second pistons have at their forward ends separate operating portions for performing operations on a common workpiece.
2. Description of the Prior Art
As is well known, actuators are widely in use in various kinds of industrial machinery for conveying workpieces, for transmitting forces and for other purposes. Emphasis is placed on the magnitude of the force for transmission, operability or responsiveness to serve the purpose of their use, and different drive systems, such as a hydraulic drive system relying on oil pressure or water pressure, a pneumatic drive system relying on air pressure or an electric drive system, are employed depending on the use to which they are put.
With an advance in the progress of factory automation achieved in recent years, there has been a strong demand for a reduction in production steps and an increase in the speed of production. Measures are being taken to perform more than two types of operations by using one type of industrial machine or to increase the speed at which a tool, for example, is moved to a location which is close to the workpiece.
Proposals have been made to meet the aforesaid demand in various ways. One apparatus developed to satisfy the demand comprises a machine tool having actuator-operated arms connected to a device for making holes and a welder respectively for performing various process steps successively. Another apparatus intended to satisfy the demand is equipped with an actuator of the two-stage operation system, as disclosed in Japanese Utility Model Examined Publication No. 5207/84, which includes a cylinder chamber having a first piston inserted therein and a second cylinder chamber formed in a piston rod of the first piston for receiving a second piston. The two cylinder chambers are selectively connectable to each other to keep them in and out of communication with each other to enable a drive fluid for the first piston to be used for driving the second piston, so as to control the speed at which a tool attached to a forward end of a piston rod of the second piston is moved to a location close to a workpiece.
Some disadvantages are associated with the apparatus of the prior art referred to hereinabove which employ actuators of the conventional type. Separate actuators should be used for different process steps, making the apparatus cumbersome and large in size. Moreover, the apparatus are low in operation efficiency because various process steps have to be followed successively.
The problem encountered in the case of the actuator of the two-stage operation system is that, since the first and second pistons share the same source of drive pressure, it is impossible to effect control of the pressure applied to each piston by altering the magnitude of the force.
Moreover, a backup force would be produced between first and second process steps depending on the use to which the apparatus is put. To cope with this situation, it would become necessary to provide a stop pin and a control mechanism. This increases costs and makes maintenance troublesome to carry out.